1. Field of the Invention
The present invention relates to a high efficiency coding technique and, more particularly, to a high efficiency coding apparatus for compressing the number of bits per picture element of video data such as a digital television signal or the like.
2. Description of the Prior Art
For coding a television signal, there are several known methods of reducing the average number of bits per picture element or lowering the sampling frequency in order to narrow the transmission band.
As one of the coding methods of lowering the sampling frequency, there is proposed the method whereby the video data is thinned out by 1/2 by subsampling, and the subsampling point and the flag indicative of the position of the subsampling point which is used upon interpolation (namely, representing which data at the sampling point in the vertical or horizontal direction of the interpolation points is used) are transmitted.
A DPCM (differential PCM) method is known as one of the coding methods of reducing the average number of bits per picture element. In the DPCM method, attention is paid to the fact that there is a high correlation among the picture elements of the video signal so that the difference between the adjacent picture elements is small, and this difference signal is digitized and transmitted.
As another coding method of reducing the average number of bits per picture element, there is known the method whereby a picture plane of one field is divided into small blocks, and the deviation between the level of the picture element at the representative point in one block and the level distribution of the data in this block is transmitted for every block.
According to the coding method of reducing the sampling frequency by subsampling, the sampling frequency is reduced by 50%, so that there is the fear of aliasing distortion.
The DPCM method has a problem in that the coding error is propagated to the subsequent coding process.
The method of coding on a block unit basis has a drawback in that a block distortion occurs at the boundary between blocks.
The present invention is intended to provide a high efficiency coding apparatus which can eliminate the problems of aliasing distortion, propagation of error, block distortion, and the like in the foregoing conventional techniques.
The applicant of the present invention has already proposed the high efficiency coding technique as disclosed in U.S. patent application Ser. No. 809,742, in which the dynamic range which is defined by the maximum and minimum values of a plurality of picture elements included in a two-dimensional or three-dimensional block is obtained and a coding process adapted to this dynamic range is performed.
Although the above technique relates to reversible coding and has the advantage that the reproduced picture quality hardly deteriorates, it has the disadvantages such that the compression rate is not very high and there is the possibility of problems in the actual transmission.